The present invention is in the field of battery technology and, more particularly, in the area of solid materials and composite materials for use in electrolytes in electrochemical cells.
Conventional lithium ion batteries include a positive electrode (or cathode as used herein), a negative electrode (or anode as used herein), an electrolyte, and, frequently, a separator. The electrolyte typically includes a liquid component that facilitates lithium ion transport and, in particular, enables ion penetration into the electrode materials.
In contrast, so-called solid-state lithium ion batteries do not include liquid in their principal battery components. Solid-state batteries can have certain advantages over liquid electrolyte batteries, such as improvements in safety because liquid electrolytes often contain volatile organic solvents. Solid-state batteries offer a wider range of packaging configurations because a liquid-tight seal is not necessary as it is with liquid electrolytes.
Generally, batteries having a solid-state electrolyte can have various advantages over batteries that contain liquid electrolyte. For small cells, such as those used in medical devices, the primary advantage is overall volumetric energy density. For example, small electrochemical cells often use specific packaging to contain the liquid electrolyte. For a typical packaging thickness of 0.5 mm, only about 60 percent of the volume can be used for the battery with the remainder being the volume of the packaging. As the cell dimensions get smaller, the problem becomes worse.
Elimination of the liquid electrolyte facilitates alternative, smaller packaging solutions for the battery. Thus, a substantial increase in the interior/exterior volume can be achieved, resulting in a larger total amount of stored energy in the same amount of space. Therefore, an all solid-state battery is desirable for medical applications requiring small batteries. The value is even greater for implantable, primary battery applications as the total energy stored often defines the device lifetime in the body.
Further, solid-state batteries can use lithium metal as the anode, thereby dramatically increasing the energy density of the battery as compared to the carbon-based anodes typically used in liquid electrolyte lithium ion batteries. With repeated cycling, lithium metal can form dendrites, which can penetrate a conventional porous separator and result in electrical shorting and runaway thermal reactions. This risk is mitigated through the use of a solid nonporous electrolyte for preventing penetration of lithium dendrites and enabling the safe use of lithium metal anodes, which directly translates to large gains in energy density, irrespective of cathode chemistry.
The electrolyte material in a solid-state lithium ion battery can be a polymer. In particular, poly(ethylene oxide) (“PEO”) can be used in forming solid polymer electrolytes. PEO has the ability to conduct lithium ions as positive lithium ions are solubilized and/or complexed by the ethylene oxide groups on the polymer chain. Solid electrolytes formed from PEO can have crystalline and amorphous regions, and it is believed that lithium ions move preferentially through the amorphous portion of the PEO material. In general, ionic conductivities on the order of 1×10−6 S/cm to 1×10−5 S/cm at room temperature can be obtained with variations on PEO-based electrolyte formulations. The electrolyte is typically formulated by adding a lithium ion salt to the PEO in advance of building the battery, which is a formulation process similar to liquid electrolytes.
However, solid-state batteries have not achieved widespread adoption because of practical limitations. For example, while polymeric solid-state electrolyte materials like PEO are capable of conducting lithium ions, their ionic conductivities are inadequate for practical power performance. Successful solid-state batteries require thin film structures, which reduce energy density. But, a battery with reduced energy density has limited utility.
Further, solid-state batteries tend to have a substantial amount or degrees of interfaces among the different solid components of the battery. The presence of such interfaces can limit lithium ion transport and impede battery performance. Interfaces can occur (i) between the domains of active material in the electrode and other components in the electrode, (ii) between the cathode and the solid electrolyte, and (iii) between the solid electrolyte and the anode structure. Poor lithium ion transport across these interfaces results in high impedance in batteries and a low capacity on charge or discharge.
A polymer solid-state electrolyte is commonly understood to have the following advantages: (i) relatively easily processed by standard solution casting or slurry casting techniques; (ii) mechanical flexibility allowing the polymer to conform to electrode surfaces, allowing for good mechanical compliance and comparatively low loss of surface contact during cycling; and (iii) relatively easy to drop in to existing lithium ion battery manufacturing with solid polymer films similar to today's separators.
However, a polymer solid-state electrolyte is also commonly understood to have the following disadvantages: (i) relatively low conductivity, on the order of 10−6-10−5 S/cm at room temperature; (ii) current state of the art polymer solid-state electrolytes typically consist of PEO-type polymers, which have poor stability at high voltage (for example, greater than about 4.2 V); and (iii) common polymer solid-state electrolytes actually form relatively soft films that are not expected to prevent lithium dendrite penetration.
In some instances, inorganic materials have been used to attempt to improve the performance of polymer solid-state electrolytes. For example, U.S. Patent Publication 2013/0026409 discloses a composite solid electrolyte with a glass or glass-ceramic inclusion and an ionically conductive polymer. However, this solid electrolyte requires a redox active additive. As another example, U.S. Pat. No. 5,599,355 discloses a method of forming a composite solid electrolyte with a polymer, salt, and an inorganic particle (such as alumina). The particles are reinforcing filler for solid electrolyte and do not transport lithium. As yet another example, U.S. Pat. No. 5,599,355 discloses a composite solid state electrolyte containing a triflate salt, PEO, and a lightweight oxide filler material. Again, the oxide filler is not a lithium ion conductor or intercalation compound.
More generally, ionically conductive polymers like PEO have been disclosed with the use of a lithium salt as the source of lithium ions in the solid electrolyte. For example, Teran et al., Solid State Ionics (2011) 18-21; Sumathipala et al., Ionics (2007) 13: 281-286; Abouimrane et al., JECS 154(11) A1031-A1034 (2007); Wang et al., JECS, 149(8) A967-A972 (2002); and Egashira et al., Electrochimica Acta 52 (2006) 1082-1086 each disclose different solid electrolyte formulations with PEO and a lithium salt as the source for lithium ions. Still further the last two references (Wang et al. and Egashira et al.) each disclose non-ion conducting inorganic nanoparticles that are believed to improve the ionic conductivity of the PEO film by preventing/disrupting polymer crystallinity. However, none of these formulations both addresses all the limitations of solid electrolytes and provides the performance improvements seen in the embodiments disclosed below.